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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic AlkaneR–CH 2 –CH 2 –RC n H 2n+2 This is the maximum H/C ratio for a given number of carbon atoms. AlkeneR–CH=CH–RC n H 2n Each double bond reduces the number of hydrogen atoms by 2. AlkyneR–C≡C–RC n H 2n-2 Each triple bond reduces the number of hydrogen atoms by 4. Each ring reduces the number of hydrogen atoms by 2

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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic In sp 2 hybridization the 2s orbital is mixed with only two of the three available 2p orbitals: forming a total of 3 sp 2 orbitals with one p-orbital remaining. In ethylene (ethene) the two carbon atoms form a σ bond by overlapping two sp 2 orbitals and each carbon atom forms two covalent bonds with hydrogen by s–sp 2 overlap all with 120° angles. The π bond between the carbon atoms perpendicular to the molecular plane is formed by 2p–2p overlap. π -bond Ethylene (ethene), showing the pi bond in green

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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic Since sp 2 hybrid orbitals are always in the same plane, the entire ethylene molecule is planar. cis -2-butene trans -2-butene Cis-/trans- isomerism are the consequence of the planarity of all atoms bonded to double-bonded (C=C) carbon atoms.

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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic Trans fatty acids turn out to increase total cholesterol levels and LDL cholesterol levels, and to reduce HDL cholesterol levels. In other words, trans fatty acids are detrimental to cardiac health. Unsaturated fatty acids become rancid relatively quickly. To combat the instability of unsaturated fatty acids, manufacturers began to "hydrogenate" them, a process that makes them more stable. The result was a more solid and longer lasting form of vegetable oil, called "partially hydrogenated" oil, in which a new type of fatty acid is formed. This new type of fatty acid is called trans fatty acid. Elaidic acid C 18 H 34 O 2

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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic Physical properties of alkenes (and alkynes) are very similar to the physical properties of alkanes with the same number of carbons (and skeletal structure).

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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic Addition of hydrogen (hydrogenation) across carbon-carbon double bond converts alkenes into alkanes. Additions of halogens (Cl 2, Br 2, I 2, etc) also called halogenation: Bromine and chlorine add readily to ethene to form 1,2-dibromoethane and 1,2-dichloroethane respectively. The reaction occurs even in the dark at room temperature.

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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic Markovnikov's Rule In the addition of an unsymmetrical reagent such as HCl, HBr and H 2 O to a double bond of an alkene, the hydrogen atom adds to the carbon of the double bond with the greatest number of hydrogen atoms. 1-butene 2-bromobutane 1-methylcyclopentene1-methylcyclopentanol

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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic Poly(ethene) (polythene or polyethylene) an example of addition polymerisation Temperature:about 200°C Pressure:about 2000 atmospheres Initiator:a small amount of oxygen as an impurity The number of molecules joining up is very variable, but is in the region of 2000 to 20000. High density poly(ethene) is used to make things like plastic milk bottles and similar containers, washing up bowls, plastic pipes and so on. Look for the letters HDPE near the recycling symbol. Low density poly(ethene) is used for familiar things like plastic carrier bags and other similar low strength and flexible sheet materials.

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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic It has uses in packaging - for example, in plastic film for shrink wrapping food. There are also medical uses - for example, in medical tubing and for medical bags and pouches. Poly(propene) (polypropylene): PP

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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic Poly(chloroethene) is commonly known by the initials of its old name (polyvinyl chloride), PVC. Poly(tetrafluoroethene): PTFE You may have come across this under the brand names of Teflon or Fluon.

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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic The chemical bonding in compounds such as alkynes with triple bonds is explained by sp hybridization. In this model the 2s orbital mixes with only one of the three p-orbitals resulting in two sp orbitals and two remaining unchanged p orbitals. The chemical bonding in acetylene (ethyne) (C 2 H 2 ) consists of sp–sp overlap between the two carbon atoms forming a σ bond and two additional π -bonds formed by p–p overlap. Each carbon also bonds to hydrogen in a sigma s–sp overlap at 180° angles. “it takes alkynes to make a world” Chemist humor

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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic acetylene (ethyne) In acetylene, the H-C≡C bond angles are 180°. By virtue of this bond angle, alkynes tend to be rod-like. sp-sp s bond Two pairs of p-orbitals forming a π -bond Acetylene is a colorless, unpleasant-smelling gas (bp - 28.1°C), which burns in air to produce a very sooty flame. In the presence of pure oxygen, however, it burns at very high temperatures (up to 2800°C), and is used in welding and cutting torches. C2H2C2H2

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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic All the C-atoms in benzene are sp 2 -hybridized. Two sp 2 -hybrid orbitals of each C-atom overlap with two sp 2 -hybrid orbital of two other C-atoms to form sigma bonds. In this way there are six sigma bonds are formed between six C-atoms which are 120 o apart. Remaining six sp 2 -orbital of six C-atoms overlap with 1s orbital of six H-atoms individually to form six sigma bonds. Since sigma bond results from the overlap of above said planar orbital, all H and C atoms are in the same plane and their generate a hexagonal ring of C-atoms. Each C-atom in benzene also has an unhybrid 2pz-orbital containing one electron. These 2pz-orbital are perpendicular to the plane of sigma bonds The overlap of these 2pz-orbital result in the formation of a fully delocalized pi-bond, which extends all over the six C-atoms of benzene nucleus. Actually these 2pz-orbital produce a pi-molecular orbital containing six electrons. One half of this pi-molecular orbital lies above the plane of hexagonal ring and remaining half below the ring like a sandwich.

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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic Why is Trinitrotoluene (TNT) Explosive? Because nitrogen in the nitro group (NO 2 ) has a charge of +1, the nitro group has a very strong tendency to withdraw (pull) electrons from the aromatic ring. Therefore, attaching three nitro groups to a benzene ring leads to an extremely unstable or explosive compound. heterocyclic aromatic compound heterocyclic aromatic compound naphthalene

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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic Benzene boils at 80°C - rather higher than other hydrocarbons of similar molecular size (pentane and hexane, for example). This is presumably due to the ease with which temporary dipoles can be set up involving the delocalized electrons. Methylbenzene (toluene) boils at 111°C. It is a bigger molecule and so the van der Waals dispersion forces will be bigger. You might have expected that methylbenzene's melting point would be higher than benzene's as well, but it isn't - it is much lower! Benzene melts at 5.5°C; methylbenzene at -95°C. Molecules must pack efficiently in the solid if they are to make best use of their intermolecular forces. Benzene is a tidy, symmetrical molecule and packs very efficiently. The methyl group sticking out in methylbenzene tends to disrupt the closeness of the packing. If the molecules aren't as closely packed, the intermolecular forces don't work as well and so the melting point falls. The arenes (aromatic compounds) are insoluble in water.

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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic Benzene is resistant to addition reactions. Adding something new to the ring would need you to use some of the delocalized electrons to form bonds with whatever you are adding. That results in a major loss of stability as the delocalization is broken. Instead, benzene mainly undergoes substitution reactions – replacing one or more of the hydrogen atoms by something new. That leaves the delocalized electrons as they were.

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UNSATURATED HYDROCARBONS Chemistry 21A Dr. Dragan Marinkovic The term carcinogen refers to any substance, radionuclide or radiation that is an agent directly involved in the promotion of cancer or in the increase of its propagation mutagen is a physical or chemical agent that changes the genetic material teratology generally refers to disfiguring birth defects or malformations Polycyclic aromatic hydrocarbons (PAHs) are chemical compounds that consist of fused aromatic rings and do not contain heteroatom or carry substituents. PAHs occur in oil, coal, and tar deposits, and are produced as byproducts of fuel burning (whether fossil fuel or biomass). As a pollutant, they are of concern because some compounds have been identified as carcinogenic, mutagenic, and teratogenic. PAHs are also found in foods. Studies have shown that most food intake of PAHs comes from cereals, oils and fats. Smaller intakes come from vegetables and cooked meats.